PURPOSE: To reduce the extension of the pulse width of a shaped waveform by constituting the smoothing circuit of an envelope detecting circuit by using a smoothing capacitor, a constant-current load circuit connected to the smoothing capacitor, and a limiter circuit.

CONSTITUTION: When a modulating signal Vi for a pulse signal is supplied from an input terminal 5, the modulating signal Vi is detected by a diode 1 and then charged in the smoothing capacitor 2. When the terminal voltage for the smoothing capacitor 2 rises above the Zener voltage of a Zener diode 7, the charging current of the smoothing capacitor 2 by-passes the smoothing capacitor 2 through the diode 7 and is not charged in the capacitor 2, so that the terminal voltage for the smoothing capacitor 2 is held at the Zener voltage. When the detection output Vd of the diode 1 drops below the Zener voltage, the charges accumulated in the smoothing capacitor 2 are discharged through the constant-voltage load circuit 8. In this case, the voltage drops linearly to 0V in a relatively short time.